Photosensitive resin composition, solder resist composition, and covered-printed wiring board

ABSTRACT

This photosensitive resin composition includes: (A) a photopolymerizable compound including at least one of a photopolymerizable monomer and a photopolymerizable oligomer; (B) titanium dioxide; and (C) a photopolymerization initiator. The component (C) includes (C1) an acylphosphine oxide-containing photopolymerization initiator and (C2) a phenylglyoxylic acid-containing photopolymerization initiator.

TECHNICAL FIELD

The present invention relates to a photosensitive resin composition, a solder resist composition and a covered-printed wiring board, and specifically relates to a photosensitive resin composition with photocurability, a solder resist composition including the photosensitive resin composition, and a covered-printed wiring board including a solder resist layer formed with the solder resist composition.

BACKGROUND ART

In recent years, as a method for forming solder resist layers on printed wiring boards for consumer use and industrial use, a developable solder resist composition with excellent resolution and dimensional accuracy has been widely used, instead of a screen printing method, in order to increase the density of wiring on the printed wiring board.

Additionally, in recent years, there is an increasing use of optical elements, such as light-emitting diodes, mounted directly on a printed wiring board covered with a solder resist layer, wherein the light-emitting diodes are often used for: backlights in liquid crystal displays for mobile terminals, personal computers, and televisions; and light sources for lighting devices. Furthermore, when titanium dioxide is contained in the solder resist layer on the printed wiring board mounted with optical elements, the solder resist layer is whitened and therefore light emitted from optical elements is efficiently reflected at the solder resist layer (see JP2012-78414A).

However, in a process of curing the solder resist composition under exposure to light, titanium dioxide contained in the solder resist composition may cause difficulty in curing of the solder resist composition due to titanium dioxide reflecting or absorbing light. Especially when the solder resist composition contains a large amount of titanium dioxide, it is difficult for the solder resist layer formed with the solder resist composition to completely cure from a surface to a deep part. If the deep part of the solder resist layer is not thoroughly cured, it is likely for defects to occur, such as lowered resolution in development, wrinkles on the solder resist layer caused by a difference in curing shrinkages of the deep part and the surface of the solder resist layer, and cracks on the solder resist layer when heated due to a partial stress caused by a difference in thermal expansion coefficients of the printed wiring board and the solder resist layer.

SUMMARY OF INVENTION

The present invention has been made in light of the above-described circumstances, and it is an object thereof to provide, a photosensitive resin composition capable of forming a cured product which is thoroughly cured from a deep part to a surface by photocuring the photosensitive resin composition, a solder resist composition including the photosensitive resin composition, and a covered-printed wiring board including a solder resist layer formed with the solder resist composition.

Solution to Problem

A photosensitive resin composition according to the present invention includes: (A) a photopolymerizable compound including at least one of a photopolymerizable monomer and a photopolymerizable oligomer; (B) titanium dioxide; and (C) a photopolymerization initiator. The component (C) includes (C1) an acylphosphine oxide-containing photopolymerization initiator and (C2) a phenylglyoxylate-containing photopolymerization initiator.

A solder resist composition according to the present invention contains the photosensitive resin composition.

A covered-printed wiring board according to the present invention includes: a printed wiring board; and a solder resist layer covering the printed wiring board. The solder resist layer includes a cured product of the solder resist composition.

According to the present invention, the photosensitive resin composition capable of forming a cured product which is thoroughly cured from a deep part to a surface by photocuring the photosensitive resin composition can be provided.

DESCRIPTION OF EMBODIMENTS

An embodiment for implementing the present invention is now described. It should be noted that in description from now on, “(meth)acryl” means at least one of “acryl” and “methacryl”. For example, (meth)acrylate means at least one of acrylate and methacrylate.

A photosensitive resin composition according to the present embodiment includes (A) a photopolymerizable compound including at least one of a photopolymerizable monomer and a photopolymerizable oligomer (also referred to as a component (A) hereinafter); (B) titanium dioxide (also referred to as a component (B) hereinafter); and (C) a photopolymerization initiator (also referred to as a component (C) hereinafter). The component (C) includes (C1) an acylphosphine oxide-containing photopolymerization initiator and (C2) a phenylglyoxylate-containing photopolymerization initiator.

The photosensitive resin composition according to the present embodiment can be cured under photoirradiation. For example, a cured film containing a cured product of the photosensitive resin composition can be obtained when the photosensitive resin composition is formed into a film and then the film of the photosensitive resin composition is irradiated with light.

The photopolymerization initiator having light absorptivity at a wavelength within a range of 380 to 420 nm can exhibit high photoactivity in a system containing a large amount of titanium dioxide. Since the component (C1) has light absorptivity at a wavelength of about 400 nm, in principle, the photosensitive resin composition containing the component (C1) is expected to have high photocurability even when titanium dioxide is contained. However, in reality, when the photosensitive resin composition contains both of titanium dioxide and the component (C1), it is difficult to cure the film of the photosensitive resin composition thoroughly under photoirradiation. Depending on a concentration of the component (C1) in the photosensitive resin composition, although a surface of the photosensitive resin composition may be fully cured, a deep part of the photosensitive resin composition may not be fully cured, and vice versa.

However, in the present embodiment, since the component (C) includes not only the component (C1) but also the component (C2), the photosensitive resin composition can be thoroughly cured from the surface to the deep part when the film of the photosensitive resin composition is irradiated with light.

Therefore, in the present embodiment, when, for example, a solder resist composition containing the photosensitive resin composition is photocured to form a solder resist layer, the solder resist layer can be thoroughly cured from the surface to the deep part.

Note that, the photosensitive resin composition according to the present embodiment is not limited to a use for the solder resist composition. The photosensitive resin composition can be applied, for example, for a photocurable printing ink.

The photosensitive resin composition according to the present embodiment will be further described specifically hereinafter.

The component (A) provides photocurability to the photosensitive resin composition. The component (A) includes at least one compound selected from a group consisting of the photopolymerizable monomer and the photopolymerizable oligomer.

The photopolymerizable monomer contains, for example, an ethylene-based unsaturated group. The photopolymerizable monomer may include at least one compound selected from a group consisting of, for example, monofunctional (meth)acrylates such as 2-hydroxyethyl (meth)acrylate and multifunctional (meth)acrylates such as diethylene glycol di(meth)acrylate, trimethylolpropane di(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol penta(meth)acrylate, dipentaerythritol hexa(meth)acrylate, and ε-caprolactone-modified dipentaerythritol hexa(meth)acrylate.

The photopolymerizable monomer preferably includes a phosphorus-containing compound (a phosphorus-containing photopolymerizable compound). In this case, flame retardancy of the cured product of the photosensitive resin composition is improved. The phosphorus-containing photopolymerizable compound may include at least one compound selected from a group consisting of, for example, 2-methacryloyloxy ethyl acid phosphate (for example, Light Ester P-1M and Light Ester P-2M manufactured by Kyoeisha Chemical Co., Ltd.), 2-acryloyloxy ethyl acid phosphate (for example, Light Acrylate P-1A manufactured by Kyoeisha Chemical Co., Ltd.), diphenyl-2-methacryloyloxy ethyl phosphate (for example, MR-260 manufactured by Daihachi Chemical Industry Co., Ltd.), and HFA series manufactured by Showa Highpolymer K. K. (for example, HFA-6003 and HFA-6007 which are products of an addition reaction of dipentaerythritol hexaacrylate and HCA, and HFA-3003 and HFA 6127 which are products of an addition reaction of caprolactone-modified dipentaerythritol hexaacrylate and HCA).

Examples of the photopolymerizable oligomer include a prepolymer which is prepared by adding an ethylene-based unsaturated group to a prepolymer obtained by polymerization of photopolymerizable monomers, and oligo (meth)acrylate prepolymers such as epoxy (meth)acrylates, polyester (meth)acrylates, urethane (meth)acrylates, alkyd resin (meth)acrylates, silicone resin (meth)acrylates, spiran resin (meth)acrylates.

The component (A) is especially preferred to include a caprolactone-modified (meth) acrylate monomer such as ε-caprolactone-modified pentaerythritol hexaacrylate. In this case, the cured product of the photosensitive resin composition is particularly effectively prevented from becoming fragile and can gain flexibility.

The component (A) may also include photopolymerizable carboxyl group-containing resin (hereinafter referred to as a component (F1)) which contains a carboxyl group and a photopolymerizable functional group. The photopolymerizable functional group is, for example, an ethylene-based unsaturated group. The component (F1) can provide the photosensitive resin composition with not only photocurability but also developability in an alkaline aqueous solution, i.e., alkali developability.

The component (F1) may include resin (hereinafter referred to as first resin (a)), for example, having a structure resulting from an addition reaction of at least one kind of a compound (a3) selected from polycarboxylic acids and anhydrides thereof, and a secondary hydroxyl group obtained from a reaction between an ethylene-based unsaturated compound (a2), which contains a carboxyl group, and at least one epoxy group in an epoxy compound (a1) which contains two or more epoxy groups per molecule.

The epoxy compound (a1) may include at least one kind of compound selected from a group consisting of, for example, cresol novolak type epoxy resin, phenol novolak type epoxy resin, bisphenol A type epoxy resin, bisphenol F type epoxy resin, bisphenol A-novolak type epoxy resin, naphthalene type epoxy resin, biphenyl type epoxy resin, biphenyl aralkyl type epoxy resin, triglycidyl isocyanurate, and alicyclic epoxy resin.

The epoxy compound (a1) may include a polymer of an ethylene-based unsaturated compound (p) which includes an epoxy group-containing compound (p1). The ethylene-based unsaturated compound (p) provided for a synthesis of the polymer may include the epoxy group-containing compound (p1) alone or in combination with an epoxy group-free compound (p2).

The epoxy group-containing compound (p1) may include a compound selected from appropriate polymers and prepolymers. Specifically, the epoxy group-containing compound (p1) may include one or more kinds of compounds selected from a group consisting of epoxy cyclohexyl derivatives of acrylic acid, epoxy cyclohexyl derivatives of methacrylic acid, an alicyclic epoxy derivative of acrylates, an alicyclic epoxy derivative of methacrylates, β-methyl glycidyl acrylate, and β-methyl glycidyl methacrylate. Especially, the epoxy group-containing compound (p1) is preferred to include glycidyl (meth)acrylate, which is widely used and easily obtained.

The epoxy group-free compound (p2) may be any compound as long as it is polymerizable with the epoxy group-containing compound (p1). The epoxy group-free compound (p2) may include one or more kinds of compounds selected from a group consisting of, for example, 2-(meth)acryloyloxyethyl phthalate, 2-(meth)acryloyloxyethyl-2-hydroxyethyl phthalate, 2-(meth)acryloyloxypropyl phthalate, benzyl (meth)acrylate, neopentyl glycol benzoate (meth)acrylate, paracumyl phenoxyethylene glycol (meth)acrylate, EO-modified cresol (meth)acrylate, ethoxylated phenyl (meth)acrylate, nonylphenoxy polyethylene glycol (meth)acrylate (polymerization degree n=2-17), ECH-modified phenoxy (meth)acrylate, phenoxy diethylene glycol (meth)acrylate, phenoxyethyl (meth)acrylate, phenoxy hexaethylene glycol (meth)acrylate, phenoxy tetraethylene glycol (meth)acrylate, tribromophenyl (meth)acrylate, EO-modified tribromophenyl (meth) acrylate, EO-modified bisphenol A di(meth)acrylate, PO-modified bisphenol A di(meth)acrylate, modified bisphenol A di(meth)acrylate, EO-modified bisphenol F di(meth)acrylate, ECH-modified phthalic acid di(meth)acrylate, trimethylolpropane benzoate (meth)acrylate, EO-modified phthalic acid (meth)acrylate, EO/PO-modified phthalic acid (meth)acrylate, vinylcarbazole, styrene, N-phenylmaleimide, N-benzylmaleimide, N-succinimidyl 3-maleimidobenzoate, straight or branched aliphatic (meth)acrylic acid esters or alicyclic (meth)acrylic acid esters (which may contain unsaturated bonding partially in a carbon ring), hydroxyalkyl (meth)acrylates, alkoxyalkyl (meth)acrylates, and N-substituted maleimides (for example, N-cyclohexylmaleimide).

The epoxy group-free compound (p2) may further include a compound which contains two or more ethylene-based unsaturated groups per molecule. When this compound is used and an amount thereof is adjusted, hardness and oiliness of the cured product of the photosensitive resin composition can be easily adjusted. The compound which contains two or more ethylene-based unsaturated groups per molecule may include one or more kinds of compounds selected from a group consisting of, for example, polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, and pentaerythritol tri(meth)acrylate.

The ethylene-based unsaturated compound (p) is polymerized to obtain polymers by a known polymerization method such as, for example, solution polymerization, and emulsion polymerization. Examples of the solution polymerization include: a method in which the ethylene-based unsaturated compound (p) is heated and stirred in presence of a polymerization initiator in an appropriate organic solvent under a nitrogen atmosphere; and azeotropic polymerization.

The organic solvent used for polymerization of the ethylene-based unsaturated compound (p) may include one or more kinds of compounds selected from a group consisting of: for example, ketones such as methyl ethyl ketone and cyclohexanone; aromatic hydrocarbons such as toluene and xylene; acetic esters such as ethyl acetate, butyl acetate, cellosolve acetate, butyl cellosolve acetate, carbitol acetate, butyl carbitol acetate, and propylene glycol monomethylether acetate; and dialkyl glycol ethers.

The polymerization initiator used for polymerization of the ethylene-based unsaturated compound (p) may include one or more kinds of compounds selected from a group consisting of, for example, hydroperoxides such as diisopropylbenzene hydroperoxide, dialkyl peroxides such as dicumyl peroxide and 2,5-dimethyl-2,5-di-(t-butylperoxy)-hexane, diacyl peroxides such as isobutyryl peroxide, ketone peroxides such as methyl ethyl ketone peroxide, alkyl peresters such as t-butyl peroxypivalate, peroxydicarbonates such as diisopropyl peroxydicarbonate, azo compounds such as azobisisobutyronitrile, and redox type initiators.

The ethylene-based unsaturated compound (a2) may include a compound selected from a group consisting of appropriate polymers and prepolymers. The ethylene-based unsaturated compound (a2) may include a compound which contains only one ethylene-based unsaturated group. The compound which contains only one ethylene-based unsaturated group may include one or more kinds of compounds selected from a group consisting of, for example, acrylic acid, methacrylic acid, crotonic acid, cinnamic acid, 2-acryloyloxyethyl succinic acid, 2-methacryloyloxyethyl succinic acid, 2-acryloyloxyethyl phthalic acid, 2-methacryloyloxyethyl phthalic acid, β-carboxyethyl acrylate, 2-acryloyloxypropyl phthalic acid, 2-methacryloyloxypropyl phthalic acid, 2-acryloyloxyethyl maleic acid, 2-methacryloyloxyethyl maleic acid, 2-acryloyloxyethyl tetrahydrophthalic acid, 2-methacryloyloxyethyl tetrahydrophthalic acid, 2-acryloyloxyethyl hexahydrophthalic acid, and 2-methacryloyloxyethyl hexahydrophthalic acid. The ethylene-based unsaturated compound (a2) may further include a compound containing a plurality of ethylene-based unsaturated groups. The compound which contains a plurality of ethylene-based unsaturated groups may include one or more kinds of compounds selected from a group consisting of compounds which are obtained by reacting a dibasic acid anhydride with a polyfunctional acrylate or a polyfunctional methacrylate which contains a hydroxyl group such as, for example, pentaerythritol triacrylate, pentaerythritol trimethacrylate, trimethylolpropane diacrylate, trimethylolpropane dimethacrylate, dipentaerythritol pentaacrylate, and dipentaerythritol pentamethacrylate.

The ethylene-based unsaturated compound (a2) is especially preferred to include at least one of acrylic acid and methacrylic acid. In this case, since an ethylene-based unsaturated group, derived from acrylic acid and methacrylic acid, has especially excellent photoreactivity, the first resin (a) can gain high photoreactivity.

An amount of the ethylene-based unsaturated compound (a2) used is preferably adjusted so that an amount of carboxyl groups in the ethylene-based unsaturated compound (a2) is within a range of 0.4 to 1.2 mol per 1 mol of epoxy groups in the epoxy compound (a1), and especially preferably adjusted so that the amount of carboxyl groups in the ethylene-based unsaturated compound (a2) is within a range of 0.5 to 1.1 mol per 1 mol of epoxy groups in the epoxy compound (a1).

The compound (a3) selected from polycarboxylic acids and anhydrides thereof may include one or more kinds of compounds selected from a group consisting of: for example, dicarboxylic acid such as phthalic acid, tetrahydrophthalic acid, methyltetrahydrophthalic acid, methylnadic acid, hexahydrophthalic acid, methylhexahydrophthalic acid, succinic acid, methylsuccinic acid, maleic acid, citraconic acid, glutaric acid, and itaconic acid; polycarboxylic acid of tri or higher basic acid such as cyclohexane-1,2,4-tricarboxylic acid, trimellitic acid, pyromellitic acid, benzophenonetetracarboxylic acid, and methylcyclohexenetetracarboxylic acid; and anhydrides thereof.

The compound (a3) is used mainly for providing the first resin (a) with an acid value and thereby supplying the photosensitive resin composition with redispersibility and resolubility in a dilute aqueous alkali solution. An amount of the compound (a3) used is adjusted so that an acid value of the first resin (a) is preferably greater than or equal to 30 mgKOH/g and especially preferably greater than or equal to 60 mgKOH/g. Furthermore, the amount of the compound (a3) used is adjusted so that an acid value of the first resin (a) is preferably less than or equal to 160 mgKOH/g and especially preferably less than or equal to 130 mgKOH/g.

In synthesis of the first resin (a), a known method can be employed to promote an addition reaction between the epoxy compound (a1) and the ethylene-based unsaturated compound (a2) and a subsequent addition reaction between a product thereof (a product of the preceding addition reaction) and the compound (a3). For example, in the addition reaction between the epoxy compound (a1) and the ethylene-based unsaturated compound (a2), the ethylene-based unsaturated compound (a2) is added to a solvent solution of the epoxy compound (a1), then if necessary a heat polymerization inhibitor and a catalyst are added, and the mixture is stirred and mixed to obtain a reactive solution. The reactive solution undergoes the addition reaction using an ordinary method at a reaction temperature of preferably 60 to 150° C. and especially preferably 80 to 120° C., and the product of the preceding addition reaction is obtained. Examples of the heat polymerization inhibitor include hydroquinone and hydroquinone monomethyl ether. Examples of the catalyst include tertiary amines such as benzyldimethylamine and triethylamine, quaternary ammonium salts such as trimethylbenzylammonium chloride and methyltriethylammonium chloride, triphenylphosphine, and triphenylstibine.

In order to promote the subsequent addition reaction between the product of the preceding addition reaction and the compound (a3), the compound (a3) is added to a solvent solution of the product of the preceding addition reaction, then if necessary a heat polymerization inhibitor and a catalyst are added, and the mixture is stirred and mixed to obtain a reactive solution. The reactive solution undergoes the addition reaction using an ordinary method, and the first resin (a) is obtained. Reaction conditions may be same as reaction conditions of the preceding addition reaction between the epoxy compound (a1) and the ethylene-based unsaturated compound (a2).

The heat polymerization inhibitor and the catalyst used for the preceding addition reaction between the epoxy compound (a1) and the ethylene-based unsaturated compound (a2), which contains a carboxyl group, may be used.

The component (F1) may include carboxyl group-containing (meth)acrylic copolymer resin (referred to as second resin (b)) obtained from a reaction between a part of carboxyl groups in a polymer of an ethylene-based unsaturated monomer including an ethylene-based unsaturated compound which contains a carboxyl group, and an ethylene-based unsaturated compound which contains an epoxy group. The ethylene-based unsaturated monomer may include an ethylene-based unsaturated compound which does not contain any carboxyl groups, if necessary.

The second resin (b) may include aromatic ring-containing (meth)acrylic copolymer resin. That is, the component (A) may include the aromatic ring-containing (meth)acrylic copolymer resin. In this case, heat resistance of the cured product of the photosensitive resin composition is especially increased. Note that the aromatic ring-containing (meth)acrylic copolymer resin is (meth)acrylic copolymer resin which contains aromatic rings. When the ethylene-based unsaturated monomer includes a compound which contains aromatic rings, the aromatic ring-containing (meth)acrylic copolymer resin is obtained.

The ethylene-based unsaturated compound, which contains a carboxyl group, used to obtain the second resin (b) may include appropriate polymers and prepolymers. For example, the ethylene-based unsaturated compound, which contains a carboxyl group, may include a compound which contains only one ethylene-based unsaturated group. Specifically, the ethylene-based unsaturated compound, which contains a carboxyl group, may include one or more kinds of compounds selected from a group consisting of, for example, acrylic acid, methacrylic acid, ω-carboxyl-polycaprolactone (n≈2) monoacrylate, crotonic acid, cinnamic acid, 2-acryloyloxyethyl succinic acid, 2-methacryloyloxyethyl succinic acid, 2-acryloyloxyethyl phthalic acid, 2-methacryloyloxyethyl phthalic acid, β-carboxyethylacrylate, 2-acryloyloxypropyl phthalic acid, 2-methacryloyloxypropyl phthalic acid, 2-acryloyloxyethyl maleic acid, 2-methacryloyloxyethyl maleic acid, 2-acryloyloxyethyl tetrahydrophthalic acid, 2-methacryloyloxyethyl tetrahydrophthalic acid, 2-acryloyloxyethyl hexahydrophthalic acid, and 2-methacryloyloxyethyl hexahydrophthalic acid. The ethylene-based unsaturated compound, which contains a carboxyl group, may also include a compound which contains a plurality of ethylene-based unsaturated groups.

Specifically, the ethylene-based unsaturated compound, which contains a carboxyl group, may include a compound obtained by reacting a dibasic acid anhydride with a polyfunctional (meth)acrylate, which contains a hydroxyl group, selected from a group consisting of, for example, pentaerythritol triacrylate, pentaerythritol trimethacrylate, trimethylolpropane diacrylate, trimethylolpropane dimethacrylate, dipentaerythritol pentaacrylate, and dipentaerythritol pentamethacrylate. These compounds may be used alone or in combination.

The ethylene-based unsaturated compound, which does not contain any carboxyl groups, used to obtain the second resin (b) may be any compound as long as it is copolymerizable with the ethylene-based unsaturated compound which contains a carboxyl group. The ethylene-based unsaturated compound, which does not contain any carboxyl groups, may include either one of an aromatic ring-containing compound and an aromatic ring-free compound. When the ethylene-based unsaturated compound, which does not contain any carboxyl groups, includes the aromatic ring-containing compound, an aromatic ring-containing (meth)acrylic copolymer resin is obtained.

The aromatic ring-containing compound may include one of more kinds of compounds selected from a group consisting of, for example, 2-(meth)acryloyloxyethyl-2-hydroxyethyl phthalate, benzyl (meth)acrylate, neopentyl glycol benzoate (meth)acrylate, paracumyl phenoxyethylene glycol (meth)acrylate, EO-modified cresol (meth)acrylate, ethoxylated phenyl (meth)acrylate, nonylphenoxy polyethylene glycol (meth)acrylate (n=2-17), ECH-modified phenoxy (meth)acrylate, phenoxy diethylene glycol (meth)acrylate, phenoxyethyl (meth)acrylate, phenoxy hexaethylene glycol (meth)acrylate, phenoxy tetraethylene glycol (meth)acrylate, tribromophenyl (meth)acrylate, EO-modified tribromophenyl (meth) acrylate, EO-modified bisphenol A di(meth)acrylate, PO-modified bisphenol A di(meth)acrylate, modified bisphenol A di(meth)acrylate, EO-modified bisphenol F di(meth)acrylate, ECH-modified phthalic acid di(meth)acrylate, trimethylolpropane benzoate (meth)acrylate, EO-modified phthalic acid (meth)acrylate, EO/PO-modified phthalic acid (meth)acrylate, N-phenylmaleimide, N-benzylmaleimide, N-vinylcarbazole, styrene, vinylnaphthalene, and 4-vinylbiphenyl.

The aromatic ring-free compound may include one or more kinds of compounds selected from a group consisting of, for example, straight or branched aliphatic (meth)acrylic acid esters or alicyclic (meth)acrylic acid esters (which may contain unsaturated bonding partially in a carbon ring), hydroxyalkyl (meth)acrylates, alkoxyalkyl (meth)acrylates, and N-substituted maleimides such as N-cyclohexylmaleimide. The aromatic ring-free compound may further include a compound, which contains two or more ethylene-based unsaturated groups per molecule, such as polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, and pentaerythritol tri(meth)acrylate. These compounds may be used alone or in combination. These compounds are preferred since hardness and oiliness of the cured product of the photosensitive resin composition can be easily adjusted.

Examples of the ethylene-based unsaturated compound, which contains an epoxy group, used to obtain the second resin (b) include appropriate polymers and prepolymers. Specific examples of the ethylene-based unsaturated compound, which contains an epoxy group, may include epoxycyclohexyl derivatives of acrylic acid or methacrylic acid, an alicyclic epoxy derivative of acrylates or methacrylates, β-methylglycidyl acrylate, and β-methylglycidyl methacrylate. These compounds may be used alone or in combination. Especially, it is preferred to use glycidyl (meth)acrylate, which is widely used and easily obtained.

The component (F1) may include resin (hereinafter referred to as third resin (C)) obtained by adding a compound, which contains an ethylene-based unsaturated group and an isocyanate group, to a part or all of hydroxyl groups in a polymer of an ethylene-based unsaturated monomer including an ethylene-based unsaturated compound, which contains a carboxyl group, and an ethylene-based unsaturated compound, which contains an epoxy group. The ethylene-based unsaturated monomer may include an ethylene-based unsaturated compound which does not contain any carboxyl groups or hydroxyl groups, if necessary.

The ethylene-based unsaturated compound, which contains a carboxyl group, used to obtain the third resin (c) may be selected from, for example, compounds which can be used as the ethylene-based unsaturated compound, which contains a carboxyl group, used to obtain the second resin (b).

Examples of the ethylene-based unsaturated compound, which contains a hydroxyl group, used to obtain the third resin (c) include: hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth) acrylate, cyclohexane dimethanol mono(meth)acrylate, 2-(meth)acryloyloxyethyl-2-hydroxyethylphthalate, caprolactone (meth)acrylate, polyethylene glycol (meth)acrylate, polypropylene glycol (meth)acrylate, trimethylolpropane di(meth)acrylate, pentaerythritol tri(meth)acrylate, and dipentaerythritol penta(meth)acrylate; hydroxybutyl vinyl ether; hydroxyethyl vinyl ether; and N-hydroxyethyl (meth)acrylamide.

Examples of the compound, which contains an ethylene-based unsaturated group and an isocyanate group, used to obtain the third resin (c) include 2-acryloyloxyethyl isocyanate (for example, KarenzAOI manufactured by Showa Denko K.K.), 2-methacryloyloxyethyl isocyanate (for example, KarenzMOI manufactured by Showa Denko K.K.), methacryloyloxy ethoxyethyl isocyanate (for example, KarenzMOI-EG manufactured by Showa Denko K.K.), isocyanate blocked compound of KarenzMOI (for example, KarenzMOI-BM manufactured by Showa Denko K.K.), isocyanate blocked compound of KarenzMOI (for example, KarenzMOI-BP manufactured by Showa Denko K.K.), and 1,1-(bisacryloyloxymethyl)ethyl isocyanate (for example, KarenzBEI manufactured by Showa Denko K.K.).

A weight-average molecular weight of the entire component (F1) is preferably within a range of 800 to 100000. Within this range, the photosensitive resin composition gains especially excellent photosensitivity and resolution.

An acid value of the entire component (F1) is preferably greater than or equal to 30 mgKOH/g. In this case, the photosensitive resin composition gains good developability. The acid value is further preferably greater than or equal to 60 mgKOH/g. In addition, the acid value of the entire component (F1) is preferably smaller than or equal to 180 mgKOH/g. In this case, an amount of remaining carboxyl groups in the cured product of the photosensitive resin composition decreases, thus maintaining good electric properties, electric corrosion resistance and water resistance of the cured product. The acid value is further preferably smaller than or equal to 150 mgKOH/g.

The photosensitive resin composition may further include a compound (hereinafter referred to as a component (F2)) which contains a carboxyl group but is not photopolymerizable. The component (F2) can provide the photosensitive resin composition with alkali developability.

The component (F2) includes, for example, a polymer of an ethylene-based unsaturated monomer including an ethylene-based unsaturated compound which contains a carboxyl group. The ethylene-based unsaturated monomer may include an ethylene-based unsaturated compound which does not contain any carboxyl groups.

The ethylene-based unsaturated compound, which contains a carboxyl group, may include appropriate polymers and prepolymers, and may include, for example, a compound which contains only one ethylene-based unsaturated group. Specifically, the ethylene-based unsaturated compound, which contains a carboxyl group, may include one or more kinds of compounds selected from a group consisting of, for example, acrylic acid, methacrylic acid, ω-carboxyl-polycaprolactone (n≈2) monoacrylate, crotonic acid, cinnamic acid, 2-acryloyloxyethyl succinic acid, 2-methacryloyloxyethyl succinic acid, 2-acryloyloxyethyl phthalic acid, 2-methacryloyloxyethyl phthalic acid, 2-acryloyloxypropyl phthalic acid, 2-methacryloyloxypropyl phthalic acid, 2-acryloyloxyethyl maleic acid, 2-methacryloyloxyethyl maleic acid, β-carboxyethylacrylate, 2-acryloyloxyethyl tetrahydrophthalic acid, 2-methacryloyloxyethyl tetrahydrophthalic acid, 2-acryloyloxyethyl hexahydrophthalic acid, and 2-methacryloyloxyethyl hexahydrophthalic acid. The ethylene-based unsaturated compound, which contains a carboxyl group, may also include a compound which contains a plurality of ethylene-based unsaturated groups. Specifically, the ethylene-based unsaturated compound, which contains a carboxyl group, may include a compound obtained by reacting a dibasic acid anhydride with a polyfunctional (meth)acrylate, which contains a hydroxyl group, selected from a group consisting of, for example, pentaerythritol triacrylate, pentaerythritol trimethacrylate, trimethylolpropane diacrylate, trimethylolpropane dimethacrylate, dipentaerythritol pentaacrylate, and dipentaerythritol pentamethacrylate. These compounds may be used alone or in combination.

The ethylene-based unsaturated compound, which does not contain any carboxyl groups, may be any compound as long as it is copolymerizable with the ethylene-based unsaturated compound which contains a carboxyl group. The ethylene-based unsaturated compound, which does not contain any carboxyl groups, may include either one of an aromatic ring-containing compound and an aromatic ring-free compound.

The aromatic ring-containing compound may include one or more kinds of compounds selected from a group consisting of, for example, 2-(meth)acryloyloxyethyl-2-hydroxyethyl phthalate, benzyl (meth)acrylate, neopentyl glycol benzoate (meth)acrylate, paracumyl phenoxyethylene glycol (meth)acrylate, EO-modified cresol (meth)acrylate, ethoxylated phenyl (meth)acrylate, nonylphenoxy polyethylene glycol (meth)acrylate (n=2-17), ECH-modified phenoxy (meth)acrylate, phenoxy diethylene glycol (meth)acrylate, phenoxyethyl (meth)acrylate, phenoxy hexaethylene glycol (meth)acrylate, phenoxy tetraethylene glycol (meth)acrylate, tribromophenyl (meth)acrylate, EO-modified tribromophenyl (meth) acrylate, EO-modified bisphenol A di(meth)acrylate, PO-modified bisphenol A di(meth)acrylate, modified bisphenol A di(meth)acrylate, EO-modified bisphenol F di(meth)acrylate, ECH-modified phthalic acid di(meth)acrylate, trimethylolpropane benzoate (meth)acrylate, EO-modified phthalic acid (meth)acrylate, EO/PO-modified phthalic acid (meth)acrylate, N-phenylmaleimide, N-benzylmaleimide, N-vinylcarbazole, styrene, vinylnaphthalene, and 4-vinylbiphenyl.

The aromatic ring-free compound may include one or more kinds of compounds selected from a group consisting of, for example, straight or branched aliphatic (meth)acrylic acid esters or alicyclic (meth)acrylic acid esters (which may contain unsaturated bonding partially in a carbon ring), hydroxyalkyl (meth)acrylates, alkoxyalkyl (meth)acrylates, and N-substituted maleimides such as N-cyclohexylmaleimide. The aromatic ring-free compound may further include a compound, which contains two or more ethylene-based unsaturated groups per molecule, such as polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, and pentaerythritol tri(meth)acrylate. These compounds may be used alone or in combination. These compounds are preferred since hardness and oiliness of the cured product of the photosensitive resin composition can be easily adjusted.

Kinds and ratios of compounds used to obtain the component (F2) appropriately selected so that an acid value of the component (F2) is an appropriate value. The acid value of the component (F2) is preferably within a range of 20 to 180 mgKOH/g and further preferably within a range of 35 to 165 mgKOH/g.

The component (B) (the titanium dioxide) colors the cured product of the photosensitive resin composition white, and accordingly the cured product can gain high light reflectivity. The component (B) may contain, for example, either one or both of rutile titanium dioxide and anatase titanium dioxide. Especially, the titanium dioxide is preferably includes the rutile titanium dioxide which has low catalyst activity and high thermal stability. The rutile titanium dioxide is manufactured industrially with a chlorine method or a sulfuric acid method. In the present embodiment, the rutile titanium dioxide may include either one or both of rutile titanium dioxide manufactured with a chlorine method and rutile titanium dioxide manufactured with a sulfuric acid method.

In the present embodiment, as described above, the component (C) (the photopolymerization initiator) includes the component (C1) (the acylphosphine oxide-containing photopolymerization initiator) and the component (C2) (the phenylglyoxylate-containing photopolymerization initiator.).

The component (C1) may include either one or both of a monoacylphosphine oxide-containing photopolymerization initiator and a bisacylphosphine oxide-containing photopolymerization initiator.

The monoacylphosphine oxide-containing photopolymerization initiator may include, for example, at least one of 2,4,6-trimethylbenzoyl-diphenyl-phosphine oxide and 2,4,6-trimethylbenzoyl-ethyl-phenyl-phosphinate.

The bisacylphosphine oxide-containing photopolymerization initiator may include one or more kinds of constituents selected from a group consisting of bis-(2,6-dichlorobenzoyl)phenylphosphine oxide, bis-(2,6-dichlorobenzoyl)-2,5-dimethylphenylphosphine oxide, bis-(2,6-dichlorobenzoyl)-4-propylphenylphosphine oxide, bis-(2,6-dichlorobenzoyl)-1-naphthylphosphine oxide, bis-(2,6-dimethoxybenzoyl)phenylphosphine oxide, bis-(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide, bis-(2,6-dimethoxybenzoyl)-2,5-dimethylphenylphosphine oxide, bis-(2,4,6-trimethylbenzoyl)phenylphosphine oxide, and (2,5,6-trimethylbenzoyl)-2,4,4-trimethylpentylphosphine oxide. Especially, the component (C1) preferably includes at least one of 2,4,6-trimethylbenzoyl-diphenyl-phosphine oxide and bis-(2,4,6-trimethylbenzoyl)phenylphosphine oxide. It is also preferable that the component (C1) includes 2,4,6-trimethylbenzoyl-diphenyl-phosphine oxide and/or bis-(2,4,6-trimethylbenzoyl)phenylphosphine oxide only. In these cases, coloring of the cured product of the photosensitive resin composition is further prevented.

The component (C2) may include at least one of (1,2-dioxo-2-methoxyethyl)benzene and a mixture of oxyphenylacetic acid 2-[2-oxo-2-phenylacetoxyethoxy]ethyl ester and oxyphenylacetic acid 2-(2-hydroxyethoxy)ethyl ester. In other words, the component (C2) may include only (1,2-dioxo-2-methoxyethyl)benzene, only a mixture of oxyphenylacetic acid 2-[2-oxo-2-phenylacetoxyethoxy]ethyl ester and oxyphenylacetic acid 2-(2-hydroxyethoxy)ethyl ester, or both of (1,2-dioxo-2-methoxyethyl)benzene and a mixture of oxyphenylacetic acid 2-[2-oxo-2-phenylacetoxyethoxy]ethyl ester and oxyphenylacetic acid 2-(2-hydroxyethoxy)ethyl ester.

In the present embodiment, no matter which of the monoacylphosphine oxide-containing photopolymerization initiator or the bisacylphosphine oxide-containing photopolymerization initiator is included in the component (C1), the photosensitive resin composition can be thoroughly cured from its surface to its deep part when the photosensitive resin composition formed into a film is irradiated with light. The reason for this is considered as follows.

The monoacylphosphine oxide-containing photopolymerization initiator has high solubility in the photosensitive resin composition but has relatively low reactivity. Accordingly, when the monoacylphosphine oxide-containing photopolymerization initiator is used alone as the photopolymerization initiator, using a high concentration of the monoacylphosphine oxide-containing photopolymerization initiator to gain good photocurability may lead to light absorption caused by a high concentration of the monoacylphosphine oxide-containing photopolymerization initiator existing in a surface part of the photosensitive resin composition, resulting in decrease in an amount of light reaching to a deep part of the photosensitive resin composition. Consequently, curability in the deep part is lowered.

On the other hand, the bisacylphosphine oxide-containing photopolymerization initiator has relatively good reactivity but has low solubility in the photosensitive resin composition. Accordingly, using a high concentration of the bisacylphosphine oxide-containing photopolymerization initiator in the photosensitive resin condition may lead to lack of surface uniformity of a coating film. When a low concentration of the bisacylphosphine oxide-containing photopolymerization initiator is used alone as the photopolymerization initiator to maintain surface uniformity of the coating film, it becomes difficult to cure the photosensitive resin composition uniformly.

However, photocurability of the photosensitive resin composition can be improved since the component (C2) can absorb light having a wavelength of about 380 to 420 nm. In addition, photocurability of the surface part of the photosensitive resin composition can be improved since the component (C2) has especially high absorptivity of light having a wavelength of around 280 nm. Furthermore, since the component (C2) can dissolve the component (C1) well, the component (C) has good dispersibility in the photosensitive resin composition.

Therefore, the photosensitive resin composition can be thoroughly cured from its surface to its deep part when the photosensitive resin composition is irradiated with light.

The component (C) may include the monoacylphosphine oxide-containing photopolymerization initiator and the component (C2). The component (C) may include only the monoacylphosphine oxide-containing photopolymerization initiator and the component (C2).

The component (C) may include the bisacylphosphine oxide-containing photopolymerization initiator and the component (C2). The component (C) may include only the bisacylphosphine oxide-containing photopolymerization initiator and the component (C2). In this case, compared to a case where the component (C1) includes only the monoacylphosphine oxide-containing photopolymerization initiator, even a low concentration of the bisacylphosphine oxide-containing photopolymerization initiator can lead to curability in the deep part of the photosensitive resin composition.

The component (C) preferably includes only the component (C1) and the component (C2). However, the component (C) may include a compound other than the component (C1) or the component (C2), without departing from the scope of the present invention. The compound other than the component (C1) or the component (C2) may include one or more kinds of compounds selected from a group consisting of: for example, benzoin and alkylethers thereof, acetophenones such as acetophenone and benzyldimethyl ketal; anthraquinones such as 2-methylanthraquinone; thioxanthones such as 2,4-dimethylthioxanthone, 2,4-diethylthioxanthone, 2-isopropylthioxanthone, 4-isopropylthioxanthone, and 2,4-diisopropylthioxanthone; benzophenones such as benzophenone and 4-benzoyl-4′-methyldiphenylsulfide; xanthones such as 2,4-diisopropylxanthone; nitrogen atom-containing compounds such as 2-methyl-1-[4-(methylthio)phenyl]-2-morpholino-1-propane; 1,2-octanedione; 1-[4-(phenylthio)-2-(O-benzoyloxyme)] (IRGACURE OXE 01); ethanone; and 1-[9-ethyl-6-(2-methylbenzoyl)-9H-carbazole-3-yl]-1-(O-acetyloxime) (IRGACURE OXE 02).

In the entire component (C), the component (C1) and the component (C2) are preferably included within a range of 3 to 100 weight % in total. In this case, the photosensitive resin composition has especially high photocurability. The component (C1) and the component (C2) are more preferably included within a range of 6 to 100 weight % in total, further preferably within a range of 9 to 100 weight %, and especially preferably within a range of 50 to 100 weight %.

In addition, in the entire component (C1) and component (C2), the component (C1) is preferably included within a range of 1 to 99 weight %. In this case, the cured product of the photosensitive resin composition can be thoroughly and highly uniformly cured from its surface to its deep part. The component (C1) is more preferably included within a range of 5 to 60 weight % and further preferably within a range of 10 to 40 weight %

The photosensitive resin composition may further include a known photopolymerization promotor and a known sensitizer. The photosensitive resin composition may include, for example, p-dimethylbenzoic acid ethylether, p-dimethylaminobenzoic acid isoamyl ester, and 2-dimethylamylethyl benzoate.

The photosensitive resin composition may include (D) an epoxy compound. In this case, the photosensitive resin composition can gain thermosettability. When the photosensitive resin composition includes the component (D), in order for the photosensitive resin composition to gain enough thermosettability, the photosensitive resin composition preferably includes a compound which contains a carboxyl group. That is the photosensitive resin composition preferably includes at least one of the component (F1) and the component (F2).

The component (D) preferably contains at least two epoxy groups per molecule. The component (D) may include either one of a hardly soluble epoxy compound and a generic soluble epoxy compound.

The component (D) may include one or more kinds of components selected from a group consisting of, for example, phenol novolak epoxy resin (for example, EPICLON N-775 manufactured by DIC Corporation), cresol novolak epoxy resin (for example, EPICLON N-695 manufactured by DIC Corporation), bisphenol A epoxy resin (for example, jER1001 manufactured by Mitsubishi Chemical Corporation), bisphenol A-novolak epoxy resin (for example, EPICLON N-865 manufactured by DIC Corporation), bisphenol F epoxy resin (for example, jER4004P manufactured by Mitsubishi Chemical Corporation), bisphenol S epoxy resin (for example, EPICLON EXA-1514 manufactured by DIC Corporation), bisphenol AD epoxy resin, biphenyl epoxy resin (for example, YX4000 manufactured by Mitsubishi Chemical Corporation), biphenyl novolak epoxy resin (for example, NC-3000 manufactured by Nippon Kayaku Co., Ltd.), hydrogenated bisphenol A epoxy resin (for example, ST-4000D manufactured by Nippon Steel & Sumikin Chemical Co., Ltd.), naphthalene epoxy resin (for example, EPICLON HP-4032, EPICLON HP-4700, EPICLON HP-4770 manufactured by DIC Corporation), hydroquinone epoxy resin (for example, YDC-1312 manufactured by Nippon Steel & Sumikin Chemical Co., Ltd.), tertiary butylcatechol epoxy resin (for example, EPICLON HP-820 manufactured by DIC Corporation), dicyclopentadiene epoxy resin (for example, EPICLON HP-7200 manufactured by DIC Corporation), adamantane epoxy resin (for example, ADAMANTATE X-E-201 manufactured by Idemitsu Kosan Co., Ltd.), biphenylether epoxy resin (for example, YSLV-80DE manufactured by Nippon Steel & Sumikin Chemical Co., Ltd.), special two-functional epoxy resin (for example, YL7175-500 and YL7175-1000 manufactured by Mitsubishi Chemical Corporation; EPICLON TSR-960, EPICLON TER-601, EPICLON TSR-250-80BX, EPICLON 1650-75MPX, EPICLON EXA-4850, EPICLON EXA-4816, EPICLON EXA-4822, and EPICLON EXA-9726 manufactured by DIC Corporation; YSLV-120TE manufactured by Nippon Steel & Sumikin Chemical Co., Ltd.), and bisphenol epoxy resins excluding the above.

The component (D) also preferably includes phosphorus-containing epoxy resin. In this case, flame retardancy of the cured product of the photosensitive resin composition is improved. Examples of the phosphorus-containing epoxy resin include phosphoric acid-modified bisphenol F epoxy resin (for example, EPICLON EXA-9726 and EPICLON EXA-97106 manufactured by DIC Corporation) and EPIKOTE FX-305 manufactured by Nippon Steel & Sumikin Chemical Co., Ltd.

The component (D) preferably includes a crystalline epoxy compound which has a melting point within a range of 130 to 160° C. In this case, tackiness of a dry coating film formed with the photosensitive resin composition is suppressed. Also in this case, when the photosensitive resin composition is dried by heating at a relatively low temperature around 60 to 80° C., even if the photosensitive resin composition includes the compound which contains a carboxyl group, the compound, which contains a carboxyl group, and the component (D) (the epoxy compound) are not easily reacted. Accordingly, the compound, which contains a carboxyl group, tends to remain unreacted in the dry coating film. Therefore, the dry coating film is first exposed to light and then developed so that high alkali developability can be assured in formation of a film. Furthermore, when the film after development is heated at an appropriate temperature, for example at 150° C., the component (D) (the epoxy compound) in the film easily softens or melts, leading to a thermosetting reaction in the film involving the component (D) (the epoxy compound). Consequently, the cured product gains high heat resistance and high hardness. The crystalline epoxy compound may include one or more kinds of compounds selected from a group consisting of, for example, triglycidyl isocyanurate, a hydroquinone epoxy compound represented by following formula (1) (such as YDC-1312 manufactured by Nippon Steel & Sumikin Chemical Co., Ltd.), and tetrakisphenol ethane crystalline epoxy resin (such as GTR-1800 manufactured by Nippon Kayaku Co., Ltd.). In the entire component (D), the crystalline epoxy compound, which has a melting point within a range of 130 to 160° C., is preferably included within 3 to 100 weight %.

R¹, R², R³, and R⁴ in the formula (1) are independently a methyl group, a hydrogen atom, or a t-butyl group

When the component (D) includes the triglycidyl isocyanurate, the triglycidyl isocyanurate is especially preferably in a form of β isomers, in which three epoxy groups are located on the same side with respect to a flat s-triazine ring, or in a mixture of β isomers and α isomers, in which one epoxy group is located on the different side from other two epoxy groups with respect to a flat s-triazine ring. When the component (D) includes the triglycidyl isocyanurate, crosslinking density in the cured product of the photosensitive resin composition is increased, leading to high hardness of the cured product. In addition, heat resistance and heat discoloration resistance of the cured product are also increased. When the component (D) includes the triglycidyl isocyanurate, the triglycidyl isocyanurate is preferably included within 3 to 100 weight % in the entire component (D).

When the component (D) includes the hydroquinone epoxy compound represented by formula (1), since the hydroquinone epoxy compound has only one aromatic ring, a long conjugated bond is less likely to occur even if the cured product of the photosensitive resin composition is decomposed due to heat or light. Furthermore, the hydroquinone epoxy compound does not contain nitrogen atom or sulfur atom. As a result, the cured product tends not to discolor. In addition, since the hydroquinone epoxy compound is bifunctional and contains an ether bond, the cured product has a reduced brittleness and thus cracks are not likely to occur when the cured product undergoes machining methods.

When the component (D) includes the hydroquinone epoxy compound, the hydroquinone epoxy compound is preferably included within 3 to 100 weight % in the entire component (D).

The photosensitive resin composition may include (E) an antioxidant. When the photosensitive resin composition includes the component (E), heat discoloration resistance of the cured product of the photosensitive resin composition is increased.

A melting point of the component (E) is preferably within a range of 50 to 150° C. With the melting point higher than or equal to 50° C., bleedout of the component (E) from the photosensitive resin composition or the film can be prevented, when the photosensitive composition is dried by heating or the film formed with the photosensitive resin composition is cured by heating. Also, with the melting point lower than or equal to 150° C., crystals of the component (E) is prevented from rising to a surface of the coating film formed with the photosensitive resin composition, resulting in prevention of lowered uniformity on a surface of the cured product.

The component (E) may include at least one kind of compound selected from a group consisting of, for example, hindered phenolic antioxidants such as IRGANOX 245 (melting point of 76 to 79° C.), IRGANOX 259 (melting point of 104 to 108° C.), IRGANOX 1035 (melting point of 63 to 67° C.), IRGANOX 1098 (melting point of 156 to 161° C.), IRGANOX 1010 (melting point of 110 to 125° C.), IRGANOX 1076 (melting point of 50 to 55° C.), and IRGANOX 1330 (melting point of 240 to 245° C.) manufactured by BASF Corporation, ADEKA STAB AO-20 (melting point of 220 to 222° C.), ADEKA STAB AO-30 (melting point of 183 to 185° C.), ADEKA STAB AO-40 (melting point of 210 to 214° C.), ADEKA STAB AO-50 (melting point of 51 to 54° C.), ADEKA STAB AO-60 (110 to 130° C.), ADEKA STAB AO-80 (110 to 120° C.), and ADEKA STAB AO-330 (melting point of 243 to 245° C.) manufactured by ADEKA CORPORATION, SEENOX224M (melting point of 129 to 132° C.) and SEENOX326M (melting point of 241 to 249° C.) manufactured by SHIPRO KASEI KAISHA, Ltd., SUMILIZER GA-80 (melting point of higher than or equal to 110° C.) and SUMILIZER MDP-S (melting point of higher than or equal to 128° C.) manufactured by Sumitomo Chemical CO., Ltd., and Antage BHT (melting point of higher than or equal to 69° C.), Antage W-300 (melting point of higher than or equal to 205° C.), Antage W-400 (melting point of higher than or equal to 120° C.), and Antage W-500 (melting point of higher than or equal to 120° C.) manufactured by Kawaguchi Chemical Industry Co., LTD. Especially, the component (E) preferably includes IRGANOX 1010 (melting point of 110 to 125° C.).

The photosensitive resin composition may include an organic solvent. The organic solvent is used to liquefy the photosensitive resin composition or form varnish with the photosensitive resin composition, and to adjust viscosity, applicability, and film-formability.

The organic solvent may include one or more kinds of compounds selected from a group consisting of, for example, straight, branched, secondary, or poly alcohols such as ethanol, propyl alcohol, isopropyl alcohol, hexanol, and ethylene glycol; ketones such as methyl ethyl ketone and cyclohexanone; aromatic hydrocarbons such as toluene and xylene; petroleum aromatic mixed solvents such as Swazol series (manufactured by Maruzen Petrochemical Co., Ltd.) and Solvesso series (manufactured by Exxon Mobil Chemical Corporation); cellosolves such as cellosolve and butyl cellosolve; carbitols such as carbitol and butyl carbitol; propylene glycol alkyl ethers such as propylene glycol methyl ether; polypropylene glycol alkyl ethers such as dipropylene glycol methyl ether; acetic acid esters such as ethyl acetate, butyl acetate, cellosolve acetate, and carbitol acetate; and dialkyl glycol ethers.

An amount of the organic solvent in the photosensitive resin composition is preferably adjusted so that the organic solvent volatilizes quickly when a coating film formed with the photosensitive resin composition is dried, i.e. the organic solvent does not remain in the dried coating film. The amount of the organic solvent in the entire photosensitive resin composition is preferably within a range of 0 to 99.5 weight %, and further preferably within a range of 15 to 60 weight %. Note that, since an appropriate amount of the organic solvent depends on a coating method, the amount is preferably adjusted appropriately depending on the coating method.

Within the scope of the present invention, the photosensitive resin composition may include components other than the above.

For example, the photosensitive resin composition may include one or more kinds of resin selected from a group consisting of blocked isocyanates of tolylene diisocyanate, morpholine diisocyanate, isophorone diisocyanate, and hexamethylene diisocyanate which are blocked with caprolactam, oxime, maloic acid ester, and the like; amino resin such as melamine resin, n-butylated melamine resin, isobutylated melamine resin, butylated urea resin, butylated melamine-urea cocondensed resin, and benzoguanamine-based cocondensed resin; various thermosetting resin other than the above; photocuring epoxy (meth)acrylate; resin obtained by adding (meth)acrylic acid to epoxy resin such as bisphenol A type epoxy resin, phenol novolak type epoxy resin, cresol novolak type expoxy resin, and alicyclic expoxy resin; and polymeric compounds such as diallyl phthalate resin, phenoxy resin, urethane resin, and fluorine resin.

When the photosensitive resin composition includes the epoxy compound (the component (D)), the photosensitive resin composition may further include a curing agent to cure the epoxy compound. The curing agent may include, for example, one or more kinds of compounds selected from a group consisting of imidazole derivatives such as imidazole, 2-methylimidazole, 2-ethylimidazole, 2-ethyl-4-methylimidazole, 2-phenylimidazole, 4-phenyl imidazole, 1-cyanoethyl-2-phenylimidazole, and 1-(2-cyanoethyl)-2-ethyl-4-methylimidazole; amine compounds such as dicyandiamide, benzyldimethylamine, 4-(dimethylamino)-N,N-dimethylbenzylamine, 4-methoxy-N,N-dimethylbenzylamine, and 4-methyl-N,N-dimethylbenzylamine; hydrazine compounds such as adipic acid hydrazide and sebacic acid hydrazide; phosphorus compounds such as triphenylphosphine; acid anhydrides; phenols; mercaptans; Lewis acid amine complexes; and onium salts. Examples of commercial products of the above compounds may include 2MZ-A, 2MZ-OK, 2PHZ, 2P4BHZ, and 2P4MHZ manufacture by Shikoku Chemicals Corporation, (which are names for commercial products of imidazole compounds), U-CAT3503N and U-CAT3502T manufactured by San-Apro Ltd., (which are names for commercial products of blocked isocyanates of dimethylamine,) and DBU, DBN, U-CATSA102, and U-CAT5002 manufactured by San-Apro Ltd., (which are bicyclic diamine compounds and salts thereof).

The photosensitive resin composition may include an adhesion imparting agent. Examples of the adhesion imparting agent may include guanamine, acetoguanamine, benzoguanamine, melamine, and S-triazine derivatives such as 2,4-diamino-6-methacryloyloxyethyl-S-triazine, 2-vinyl-4,6-diamino-S-triazine, 2-vinyl-4,6-diamino-S-triazine-isocyanuric acid adduct, and 2,4-diamino-6-methacryloyloxyethyl-S-triazine-isocyanuric acid adduct.

The photosensitive resin composition may include one or more kinds of constituents selected from a group consisting of a curing promotor; a coloring agent other than white; a copolymer such as silicones and acrylates; a leveling agent; an adhesion imparting agent such as silane coupling agents; a thixotropy agent; a polymerization inhibitor; a halation preventer; a flame retardant; a defoaming agent; a surfactant; a polymer dispersant; and an inorganic filler such as barium sulfate, crystalline silica, nano silica, carbon nanotube, talc, bentonite, aluminum hydroxide, and magnesium hydroxide.

An amount of each component in the photosensitive resin composition is appropriately adjusted so that the photosensitive resin composition has photocurability and is developable with an alkaline aqueous solution.

In the photosensitive resin composition, the amount of the component (A) is preferably within a range of 5 to 95 weight %, more preferably within a range of 10 to 90 weight %, and further preferably within a range of 10 to 40 weight %, with respect to solid content of the photosensitive resin composition.

In the photosensitive resin composition, the amount of the component (B) is preferably within a range of 15 to 500 parts by mass, with respect to 100 parts by mass of the component (A). Furthermore, in the photosensitive resin composition, the amount of the component (B) is preferably within a range of 3 to 220 weight % and further preferable within a range of 10 to 180 weight %, with respect to resin content of the photosensitive resin composition.

In the photosensitive resin composition, the amount of the component (C) is preferably within a range of 0.1 to 30 weight % and further preferably within a range of 1 to 28 weight %, with respect to solid content of the photosensitive resin composition.

When the photosensitive resin composition includes the component (D), in the photosensitive resin composition, the amount of the component (D) is preferably within a range of 1.5 to 85 weight %, more preferably within a range of 1.5 to 60 weight %, and further preferably within a range of 2 to 40 weight %, with respect to solid content of the photosensitive resin composition.

In the photosensitive resin composition, the amount of the component (E) is preferably within a range of 0.005 to 15 weight % and further preferably within a range of 0.05 to 10 weight %, with respect to solid content of the photosensitive resin composition.

When the photosensitive resin composition includes at least one of the component (F1) and the component (F2), in the photosensitive resin composition, the total amount of the component (F1) and the component (F2) is preferably within a range of 5 to 85 weight %, more preferably within a range of 10 to 75 weight %, and further preferably within a range of 10 to 40 weight %, with respect to solid content of the photosensitive resin composition. Note that, the solid content of the photosensitive resin composition is defined as a total amount of all components included in the photosensitive resin composition except for components such as solvents which volatilizes in formation of the cured product from the photosensitive resin composition. In addition, the resin content of the photosensitive resin composition is defined as a total amount of the component (A), the component (D), the component (F1), and the component (F2) included in the photosensitive resin composition.

Ingredients as described above for the photosensitive resin composition are combined and kneaded by a known kneading method using, for example, a three-roll, a ball mill, or a sand mill to obtain the photosensitive resin composition.

In regard to preservation stability of the photosensitive resin composition, some of the ingredients of the photosensitive resin composition may be mixed to obtain a first mixture, and rest of the ingredients may be mixed to obtain a second mixture. That is, the photosensitive resin composition may include the first mixture and the second mixture. For example, photopolymerizable compounds, some of organic solvents, and epoxy compounds out of all the ingredients may be mixed and dispersed in advance to obtain the first mixture, and the rest of the ingredients may be mixed and dispersed to obtain the second mixture. In this case, required amounts of the first mixture and the second mixture may be mixed to obtain a mixture which is used to form the cured product of the photosensitive resin composition.

A solder resist composition according to the present embodiment includes the photosensitive resin composition. The solder resist composition may include the photosensitive resin composition only. The solder resist composition is applied, for example, to form a solder resist layer on a printed wiring board.

Described below is an example of a method to form the solder resist layer on the printed wiring board using the solder resist composition according to the present embodiment. In this example, a solder resist layer is formed with a solder resist composition which has photocurability and thermosettability.

First, a printed wiring board is prepared, and a coating film of the solder resist composition is formed on the printed wiring board. For example, a surface of the printed wiring board is coated with the solder resist composition to form the coating film in a wet state (the wet coating film). A coating method to coat the solder resist composition is selected from a group consisting of known methods such as, for example, a dipping method, a spray method, a spin coating method, a roll coating method, a curtain coating method, and a screen printing method. Subsequently, if necessary, in order for the organic solvent in the solder resist composition to volatilize, the wet coating film is dried at a temperature, for example, within a range of 60 to 120° C. to obtain the coating film after drying (the dry coating film). In the present embodiment, tackiness of the dry coating film is suppressed since the photopolymerization initiator includes three specific kinds of components as described above.

Note that, in formation of the coating film on the printed wiring board, the solder resist composition may be applied to an appropriate supporting body and dried to form the dry coating film. The dry coating film may be then stacked on the printed wiring board, and pressure is applied to the dry coating film and the printed wiring board to form the dry coating film on the printed wiring board (a dry film method).

Subsequently, a negative mask is placed either directly or indirectly on the dry coating film on the printed wiring board and then light is irradiated to the negative mask so that the coating film is exposed to light through the negative mask. The negative mask includes an exposed part, which transmits light, and an unexposed part, which does not transmit light. The exposed part of the negative mask has a shape corresponding to a pattern shape of the solder resist layer. For example, photo tools such as a mask film and a dry plate are used as the negative mask. Light for exposure is selected depending on composition of the solder resist composition, and ultraviolet rays are used in the present embodiment. A light source for ultraviolet rays is selected from a group consisting of, for example, a chemical lamp, a low pressure mercury lamp, a medium pressure mercury lamp, an ultra-high pressure mercury lamp, a xenon lamp, a LED, and a metal halide lamp.

Note that, a method which does not use a negative mask may be employed as an exposure method. For example, a direct drawing method such as laser exposure may be employed. An appropriate printing method such as a screen printing method, an offset printing method, and an ink jet printing method may also be used for application of the solder resist composition to form an appropriately patterned coating film which is then exposed to light.

In the present embodiment, when the dry coating film is exposed to ultraviolet rays, a photocuring reaction proceeds efficiently in the dry coating film from its surface part to its deep part.

After the dry coating film is exposed to light, the negative mask is removed from the printed wiring board and then the dry coating film undergoes a development process to remove the unexposed part of the dry coating film. Accordingly, the exposed part of the dry coating film remains as the solder resist layer on a first surface and a second surface of the printed wiring board.

In the development process, an appropriate developer depending on the composition of the solder resist composition may be used. Examples of the developer may include alkaline solutions such as aqueous solutions of sodium carbonate, potassium carbonate, ammonium carbonate, sodium hydrogen carbonate, potassium hydrogen carbonate, ammonium hydrogen carbonate, sodium hydroxide, potassium hydroxide, ammonium hydroxide, tetramethylammonium hydroxide, and lithium hydroxide. Organic amines such as monoethanolamine, diethanolamine, triethanolamine, monoisopropanolamine, di isopropanolamine, and triisopropanolamine may be used as the developer. Above described developers may be used alone or in combination. When the developer is the alkaline solution, a solvent of the alkaline solution may be water alone or may be a mixture of water and a hydrophilic organic solvent such as lower alcohols.

When the solder resist composition includes a thermosetting component, the solder resist layer may be thermosetted with heating treatment, if necessary. As for conditions of the heating treatment, for example, a heating temperature is within a range of 120 to 180° C. and a heating period is within a range of 30 to 90 minutes. Accordingly, properties of the solder resist layer, such as strength, hardness, and chemical resistance, are improved.

Furthermore, after the solder resist layer undergoes the heating treatment, the solder resist layer may be irradiated with ultraviolet rays if necessary. In this case, the photocuring reaction further proceeds in the solder resist layer. Accordingly, migration resistance of the solder resist layer is further improved.

As a result, a covered-printed wiring board which includes the printed wiring board and the solder resist layer covering the printed wiring board is obtained. In the present embodiment, the solder resist layer can be cured from its surface part to its deep part.

EXAMPLES

Hereinafter, examples of the present invention are described. Note that, the present invention is not limited to following examples.

[Preparation of a Photopolymerizable Compound] (1) Preparation of a Photopolymerizable Oligomer A

75 parts by mass of methacrylic acid, 85 parts by mass of methyl methacrylate, 20 parts by mass of styrene, 20 parts by mass of butyl methacrylate, 440 parts by mass of dipropylene glycol monomethyl ether, and 5 parts by mass of azobisisobutyronitrile were added to a four-neck flask attached with a reflux condenser, a thermometer, a glass tube for nitrogen-substitution, and a stirrer. A mixture in the four-neck flask was heated at 75° C. for 5 hours under a nitrogen gas stream for a polymerization reaction to proceed, resulting in a 31% copolymer solution.

0.1 parts by mass of hydroquinone, 70 parts by mass of 3,4-epoxycyclohexylmethyl methacrylate, and 0.8 parts by mass of dimethylbenzylamine were added to the copolymer solution and then heated at 80° C. for 24 hours for an addition reaction to proceed. As a result, a 38% solution of a compound, which contains a carboxyl group and an ethylene-based unsaturated group, was obtained. The solution had 110 mgKOH/g of an acid value of a solid component. The solution was defined as the photopolymerizable oligomer A.

(2) Preparation of a Photopolymerizable Oligomer B

70 parts by mass of methacrylic acid, 100 parts by mass of methyl methacrylate, 30 parts by mass of t-butyl methacrylate, 440 parts by mass of dipropylene glycol monomethyl ether, and 5 parts by mass of azobisisobutyronitrile were added to a four-neck flask attached with a reflux condenser, a thermometer, a glass tube for nitrogen-substitution, and a stirrer. A mixture in the four-neck flask was heated at 75° C. for 5 hours under a nitrogen gas stream for a polymerization reaction to proceed, resulting in a 31% copolymer solution.

0.1 parts by mass of hydroquinone, 70 parts by mass of 3,4-epoxycyclohexylmethyl methacrylate, and 0.8 parts by mass of dimethylbenzylamine were added to the copolymer solution and then heated at 80° C. for 24 hours for an addition reaction to proceed. As a result, a 38% solution of a compound, which contains a carboxyl group and an ethylene-based unsaturated group, was obtained. The solution had 98 mgKOH/g of an acid value of a solid component. The solution was defined as the photopolymerizable oligomer B.

(3) Preparation of a Photopolymerizable Oligomer C

An acid-modified epoxy acrylate solution (Ripoxy PR-300CP, concentration of 65%, manufactured by Showa Denko K.K.) was prepared and used as the photopolymerizable oligomer C.

[Preparation of a Solder Resist Composition]

A mixture obtained by mixing components listed in tables below was kneaded with a three-roll to obtain the solder resist composition. Note that, details of the components listed in the tables are as following.

-   -   Photopolymerization initiator (IRGACURE TPO):         2,4,6-trimethylbenzoyl-diphenyl-phosphine oxide, manufactured by         BASF, item No. IRGACURE TPO.     -   Photopolymerization initiator (IRGACURE 819):         bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide, manufactured         by BASF, item No. IRGACURE 819.     -   Photopolymerization initiator (IRGACURE MBF):         (1,2-dioxo-2-methoxyethyl)benzene, manufactured by BASF, item         No. IRGACURE MBF.     -   Photopolymerization initiator (IRGACURE 754): a mixture of         oxyphenylacetic acid 2-[2-oxo-2-phenylacetoxyethoxy]ethyl ester         and oxyphenylacetic acid 2-(2-hydroxyethoxy)ethyl ester,         manufactured by BASF, item No. IRGACURE 754.     -   Photopolymerization initiator (IRGACURE 1173):         2-hydroxy-2-methyl-1-phenyl-propane-1-one, manufactured by BASF,         item No. IRGACURE 1173.     -   Photopolymerization initiator (IRGACURE 184):         1-hydroxy-cyclohexyl-phenyl-ketone, manufactured by BASF, item         No. IRGACURE 184.     -   Titanium dioxide CR-90: rutile titanium dioxide manufactured         with a chlorine method, manufactured by ISHIHARA SANGYO KAISHA,         LTD., item No. CR-90.     -   Titanium dioxide R-79: rutile titanium dioxide manufactured with         a sulfuric acid method, manufactured by SAKAI CHEMICAL INDUSTRY         CO., LTD., item No. R-79.     -   Epoxy compound YDC-1312: a hydroquinone epoxy compound         represented by formula (1) (2,5-di-tert-butylhydroquinone         diglycidyl ether), manufactured by Nippon Steel & Sumikin         Chemical Co., Ltd., item No. YDC-1312, melting point at 145° C.     -   Epoxy compound TGIC: triglycidyl isocyanurate         (1,3,5-tris(2,3-epoxypropyl)-1,3,5-triazine-2,4,6(1H, 3H,         5H)-trione (a high melting point type)), melting point at 158°         C.     -   EPICLON 850: bisphenol A epoxy resin, manufactured by DIC         Corporation, item No. EPICLON 850.     -   75% solution of EHPE-3150: a 75% solution of         1,2-epoxy-4-(2-oxiranyl)cyclohexane adduct of         2,2-bis(hydroxymethyl)-1-butanol.     -   Antioxidant IRGANOX 1010: a hindered phenolic antioxidant,         manufactured by BASF, item No. IRGANOX 1010, melting point at         115° C.     -   Antioxidant IRGANOX 1330: a hindered phenolic antioxidant,         manufactured by BASF, item No. IRGANOX 1330, melting point at         242° C.     -   Organic solvent: methylpropylene diglycol, manufactured by         Nippon Nyukazai Co., Ltd., item No. MFDG.     -   Photopolymerizable monomer DPHA: dipentaerythritol hexaacrylate,         manufactured by Nippon Kayaku Co., Ltd., item No. KAYARAD DPHA.     -   Photopolymerizable monomer DPCA-60: caprolactone-modified         (meth)acrylate monomer, manufactured by Nippon Kayaku Co., Ltd.,         item No. KAYARAD DPCA-60.     -   Defoaming agent: manufactured by Shin-Etsu Chemical Co., Ltd.,         item No. KS-66.     -   Melamine: manufactured by NISSAN CHEMICAL INDUSTRIES, Ltd.,         fine-particulate melamine.

[Evaluation Tests] (1) Preparation of Test Pieces

A glass epoxy copper-clad laminated plate including a copper foil with thickness of 35 μm was prepared. A conductor wiring was formed by etching on the glass epoxy copper-clad laminated plate to obtain a printed wiring board. A surface of the obtained printed wiring board was entirely coated with the solder resist composition by a screen printing method and thereby a coating film was formed. The coating film was heated at 80° C. for 20 minutes and dried. Thickness of the coating film after drying (the dry coating film) was 20 μm. With a negative mask placed directly on the dry coating film, the negative mask was irradiated with ultraviolet rays with an exposing device equipped with a metal halide lamp, and accordingly the dry coating film was selectively exposed to light with 450 mJ/cm² of exposure. Then, the negative mask was removed from the dry coating film, and the dry coating film was developed with a sodium carbonate aqueous solution so that a part of the dry coating film, which was cured due to exposure to light, remains as a solder resist layer on the printed wiring board. The solder resist layer was further heated at 150° C. for 60 minutes and thermosetted. As a result, a test piece including the solder resist layer was obtained.

Following evaluation tests were carried out for each test piece.

(2) Evaluation of Tackiness

In preparation of each test piece, when the negative mask was removed from the dry coating film after exposure to light, peeling resistance between the dry coating film and the negative mask and a condition of the dry coating film after removal of the negative mask were observed. The results were evaluated as follows.

A: No stickiness was observed when the dry coating film before exposure to light was touched with a finger, and no traces of the negative mask were observed on the dry coating film after removal of the negative mask succeeding to exposure to light. B: Slight stickiness was observed when the dry coating film before exposure to light was touched with a finger, and traces of the negative mask were observed on the dry coating film after removal of the negative mask succeeding to exposure to light. C: Conspicuous stickiness was observed when the dry coating film before exposure to light was touched with a finger, and the dry coating film was damaged after removal of the negative mask succeeding to exposure to light.

(3) Evaluation of Remained Solder Dam

A printed wiring board including a copper conductor wiring with line width of 0.2 mm, line interval of 0.3 mm, and thickness of 40 μm was prepared. A negative mask, which has a mask pattern to form solder dams with four different widths of 50 μm, 75 μm, 100 μm, and 125 μm, was used. Under same conditions as preparation of the test pieces other than using the above printed wiring board and the above negative mask, a solder dam with thickness of 40 μm was formed on the printed wiring board.

Peeling test was carried out on the solder dam using a cellophane adhesion tape, and a minimum width of remained solder dam, which was not peeled off, on the printed wiring board was measured. It is evaluated that the smaller the minimum width is, the higher curing degree is in a deep part of the solder dam.

(4) Evaluation of Photosensitivity (Remained Steps)

A test mask for exposure to light (Step Tablet PHOTEC 21 steps manufactured by Hitachi Chemical Co., Ltd.) was directly placed and attached by low pressure adhesion on the dry coating film formed with a liquid solder resist composition of each example and comparative example. Then, the dry coating film was irradiated with ultraviolet rays with irradiation energy density of 450 mJ/cm² through the test mask, using a both-side exposing device of low pressure adhesion type manufactured by ORC Manufacturing Co., Ltd. (model No. ORCHMW680GW) equipped with a metal halide lamp. The dry coating film was then developed with a developer (a sodium carbonate aqueous solution with concentration of 1 weight %). Photosensitivity of the dry coating film was evaluated in terms of the number of remained steps.

(5) Evaluation of Crack Resistance

Each test piece was cut off with a utility knife, and then peeling test was carried out on the solder resist layer close to a cut face using a cellophane adhesion tape. The solder resist layer was observed after peeling test. The results were evaluated as follows.

A: No crack was observed on the solder resist layer, and the solder resist layer was not peeled off after peeling test using a cellophane adhesion tape. B: Slight crack was observed on the solder resist layer, but the solder resist layer was not peeled off after peeling test using a cellophane adhesion tape. C: Substantial crack was observed on the solder resist layer, but the solder resist layer was not peeled off after peeling test using a cellophane adhesion tape. D: The solder resist layer was peeled off after peeling test using a cellophane adhesion tape.

(6) Evaluation of Surface Appearance

Surface appearance of the solder resist layer of each test piece was observed visually. The results were evaluated as follows.

A: No defect such as fine particles and bleeding was observed, and a surface of the solder resist layer was uniform. B: Defect such as fine particles and bleeding was observed, or a surface of the solder resist layer was not uniform with unevenness and uneven gloss.

(7) Evaluation of Solder Heat Resistance

A flux was applied on the solder resist layer of each test piece using LONCO 3355-11 (a water-soluble flux manufactured by London Chemical Co., Inc.). Succeedingly, each test piece was dipped in a melted solder bath at 260° C. for 10 seconds and then rinsed with water, which is defined as one process. After the process was carried out 3 times, surface appearance of the solder resist layer was observed. The results were evaluated as follows.

A: No change was observed. B: Very slight change was observed. C: Slight change was observed. D: Substantial change such as peeling of the solder resist layer was observed.

(8) Evaluation of Heat Discoloration Resistance

A b* value in L*a*b* color system was measured for the solder resist layer of each test piece right after preparation, using a spectral colorimeter manufactured by KONICA MINOLTA SENSING, INC. (model No. CM-600d). Succeedingly, each test piece was heated at 250° C. for 5 minutes and then a b* value of the solder resist layer was measured again. A value (Δb*) was calculated by subtracting the b* value of the solder resist layer before heating from the b* value of the solder resist layer after heating. The results were evaluated as follows.

A: The Δb* value was less than 2.0. B: The Δb* value was larger than or equal to 2.0 and less than 2.5. C: The Δb* value was larger than or equal to 2.5 and less than 3.0. D: The Δb* value was larger than or equal to 3.0.

(9) Evaluation of Light Discoloration Resistance

A b* value in L*a*b* color system was measured for the solder resist layer of each test piece, using a spectral colorimeter manufactured by KONICA MINOLTA SENSING, INC. (model No. CM-600d). Succeedingly, the solder resist layer of each test piece was irradiated with ultraviolet rays under a condition of 50 J/cm² using an exposing device equipped with a metal halide lamp, and then a b* value, in L*a*b* color system, of the solder resist layer was measured again. A value (Δb*) was calculated by subtracting the b* value of the solder resist layer before ultraviolet irradiation from the b* value of the solder resist layer after ultraviolet irradiation. The results were evaluated as follows.

A: The Δb* value was less than 2.0. B: The Δb* value was larger than or equal to 2.0 and less than 2.5. C: The Δb* value was larger than or equal to 2.5 and less than 3.0. D: The Δb* value was larger than or equal to 3.0.

(10) Reflectance

A stimulus value Y expressing luminous reflectance in CIE color system was measured for the solder resist layer of each test piece. The Y value is defines as an alternative characteristic of reflectance. A spectral colorimeter manufactured by KONICA MINOLTA SENSING, INC. with model No. CM-600d was used. For standardization, a standard white surface with a known spectral reflectance factor was used.

(11) Evaluation of Adhesion

Conforming to a test method of JIS D0202, the solder resist layer of each test piece was cross-cut into a checker board pattern and peeling test using a cellophane adhesion tape was carried out. The solder resist layer after peeling test was observed visually. The results were evaluated as following.

A: No change was observed in all of 100 cross-cut sections. B: Floating of the solder resist layer was observed in one of 100 cross-cut sections. C: Peeling of the solder resist layer was observed in 2 to 10 of 100 cross-cut sections. D: Peeling of the solder resist layer was observed in 11 to 100 of 100 cross-cut sections.

(12) Pencil Hardness

Pencil hardness of the solder resist layer of each test piece was measured using Mitsubishi Hi-Uni pencils (manufactured by Mitsubishi Pencil Co., Ltd.), conforming to JIS K5400.

TABLE 1 Examples 1 2 3 4 5 6 7 8 9 10 11 12 13 Composition/ Photopolymerizable oligomer A 80 80 80 80 80 80 80 80 80 80 80 80 80 parts by mass Photopolymerizable oligomer B Photopolymerizable oligomer C Photopolymerization initiator 6 (IRGACURE TPO) Photopolymerization initiator 4 4 4 3 4 4 4 4 4 4 4 4 (IRGACURE 819) Photopolymerization initiator 10 10 10 10 5 10 10 10 10 10 10 10 (IRGACURE MBF) Photopolymerization initiator 15 (IRGACURE 754) Photopolymerization initiator 5 (IRGACURE 1173) Photopolymerization initiator 6 (IRGACURE 184) Titanium dioxide CR-90 60 60 30 60 60 60 60 60 60 60 60 60 Titanium dioxide R-79 30 60 Epoxy compound YDC-1312 15 15 15 15 15 15 15 15 10 15 Epoxy compound TGIC 15 5 10 Epiclon 850 5 75% solution of EHPE-3150 20 Antioxidant IRGANOX 1010 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Antioxidant IRGANOX 1330 Organic solvent 10 10 10 10 10 10 10 10 10 10 10 10 10 Photopolymerizable monomer DPHA 5 5 5 5 5 5 5 5 5 5 5 5 10 Photopolymerizable monomer 5 5 5 5 5 5 5 5 5 5 5 5 DPCA-60 Defoamer 2 2 2 2 2 2 2 2 2 2 2 2 2 Melamine 2 2 2 2 2 2 2 2 2 2 2 2 2 Evaluation Tackiness A A A A B A A A A A B B A Remained solder dam (μm) 75 50 50 50 75 75 50 50 50 50 50 50 50 Photosensitivity 8 8 8 8 8 8 9 8 8 8 7 6 8 Crack resistance A A A B A A A A C B C D A Surface appearance A A A A A A A A A A A A A Solder heat resistance A A A A A A A A A A A A A Heat discoloration resistance A A A C A A A A B A B A A Light discoloration resistance A A A A A A A A B A B A A Reflectance 80 80 81 80 80 80 80 81 81 81 79 80 80 Adhesion A A A A A A A A A A A B A Pencil hardness 5H 5H 5H 5H 5H 5H 5H 5H 5H 5H 5H 4H 5H

TABLE 2 Examples Comperative Examples 14 15 16 17 18 19 20 1 2 3 4 5 6 Composition/ Photopolymerizable oligomer A 80 80 80 80 80 80 80 80 80 80 80 parts by mass Photopolymerizable oligomer B 80 Photopolymerizable oligomer C 80 Photopolymerization initiator 4 4 6 12 6 6 (IRGACURE TPO) Photopolymerization initiator 4 4 4 4 4 4 4 4 4 4 (IRGACURE 819) Photopolymerization initiator 10 10 10 10 10 5 10 15 (IRGACURE MBF) Photopolymerization initiator (IRGACURE 754) Photopolymerization initiator 5 (IRGACURE 1173) Photopolymerization initiator (IRGACURE 184) Titanium dioxide CR-90 60 60 60 60 60 60 60 60 60 60 60 60 60 Titanium dioxide R-79 Epoxy compound YDC-1312 15 15 15 15 5 15 15 15 15 10 10 Epoxy compound TGIC 15 5 15 5 5 Epiclon 850 75% solution of EHPE-3150 10 Antioxidant IRGANOX 1010 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Antioxidant IRGANOX 1330 0.3 1.5 Organic solvent 10 10 10 10 10 10 10 10 10 10 10 10 Photopolymerizable monomer DPHA 10 5 5 5 5 5 5 5 5 5 5 5 Photopolymerizable monomer 5 5 5 5 5 10 5 5 5 5 5 5 DPCA-60 Defoamer 2 2 2 2 2 2 2 2 2 2 2 2 2 Melamine 2 2 2 2 2 2 2 2 2 2 2 2 2 Evaluation Tackiness A A A A A A A A A A B A A Remained solder dam (μm) 75 50 50 50 50 75 75 125 100 50 125 50 50 Photosensitivity 8 8 8 9 10 8 7 5 10 6 5 9 9 Crack resistance C A A A A B B A A A A B B Surface appearance A A A A A A A A A B A B B Solder heat resistance A A A A A A A A A A A A A Heat discoloration resistance A B A C A A A A A A A A A Light discoloration resistance A A A C A A A A A A A A A Reflectance 80 80 80 76 80 80 80 79 80 79 78 80 80 Adhesion A A A A A A A A A A A A A Pencil hardness 5H 5H 5H 6H 5H 5H 5H 5H 5H 5H 5H 5H 5H 

1. A photosensitive resin composition comprising: (A) a photopolymerizable compound including at least one of a photopolymerizable monomer and a photopolymerizable oligomer; (B) titanium dioxide; and (C) a photopolymerization initiator, the component (C) including (C1) an acylphosphine oxide-containing photopolymerization initiator and (C2) a phenylglyoxylic acid-containing photopolymerization initiator.
 2. The photosensitive resin composition according to claim 1, wherein the component (A) includes (F1) a compound which contains a carboxyl group.
 3. The photosensitive resin composition according to claim 2, wherein the component (F1) includes a (meth)acrylic copolymer.
 4. The photosensitive resin composition according to claim 1, wherein the component (A) includes a caprolactone-modified (meth)acrylate monomer.
 5. The photosensitive resin composition according to claim 1, wherein the component (B) includes rutile titanium dioxide.
 6. The photosensitive resin composition according to claim 1, wherein the component (C1) includes a bisacylphosphine oxide-containing photopolymerization initiator.
 7. The photosensitive resin composition according to claim 1 comprising (D) an epoxy compound.
 8. The photosensitive resin composition according to claim 7, wherein the component (D) includes a crystalline epoxy compound that has a melting point within a range of 130 to 160° C.
 9. The photosensitive resin composition according to claim 7, wherein: the component (D) includes at least one of triglycidyl isocyanurate and a hydroquinone epoxy compound represented by following formula (1); and R¹, R², R³, and R⁴ in the formula (1) are independently a methyl group, a hydrogen atom, or a t-butyl group.
 10. The photosensitive resin composition according to claim 1 further comprising (E) an antioxidant.
 11. The photosensitive resin composition according to claim 10, wherein the component (E) has a melting point within a range of 50 to 150° C.
 12. A solder resist composition containing the photosensitive resin composition according to claim
 1. 13. A covered-printed wiring board including: a printed wiring board; and a solder resist layer covering the printed wiring board, the solder resist layer containing a cured product of the solder resist composition according to claim
 12. 